alance, and never
by weights?

_A._--They are never pressed down by weights; in fact, weights would not
answer on a locomotive at all, as they would jump up and down with the
jerks or jolts of the train, and cause much of the steam to escape. In land
and marine boilers, however, the safety valve is always kept down by
weights; but in steam vessels a good deal of steam is lost in stormy
weather by the opening of the valve, owing to the inertia of the weights
when the ship sinks suddenly in the deep recess between the waves.

316. _Q._--What other sizes of safety valves are used in locomotives?

_A._--Some are as large as 4 inches diameter, giving 12 square inches of
area; and others are as small as 1-3/16 inch diameter, giving 1 square inch
of area.

317. _Q._--And are these valves all pressed down by a Salter's spring
balance?

_A._--In the great majority of cases they are so, and the lever by which
they are pressed down is generally graduated in the proportion of the area
of the valve to unity; that is, in the case of a valve of 12 inches area,
the long end of the lever to which the spring balance is attached is 12
times the length of the short end, so that the weight or pressure on the
balance shows the pressure per square inch on the boiler. In some cases,
however, a spiral spring, and in other cases a pile of elliptical springs,
is placed directly upon the top of the valve, and it appears desirable that
one of the valves at least should be loaded in this manner. It is difficult
when the lever is divided in such a proportion as 12 to 1, to get
sufficient lift of the valve without a large increase of pressure on the
spring; and it appears expedient, therefore, to employ a shorter lever,
which involves either a reduction in the area of the valve, or an increased
strength in the spring.

318. _Q._--What are the proper dimensions of the steam passages?

_A._--In slow working engines the common size of the cylinder passages is
one twenty-fifth of the area of the cylinder, or one fifth of the diameter
of the cylinder, which is the same thing. This proportion corresponds very
nearly with one square inch per horse power when the length of the cylinder
is about equal to its diameter; and one square inch of area per horse power
for the cylinder ports and eduction passages answers very well in the case
of engines working at the ordinary speed of 220 feet per minute. The area
of the steam pipe is usually made less than th